An LTE-A (Long Term Evolution-Advanced, Long Term Evolution-Advanced) system is a system further evolved and enhanced from a 3GPP (3rd Generation Partnership Project, 3rd Generation Partnership Project) LTE system. In the LTE-A system, to meet a requirement of the International Telecommunication Union for a peak data rate of fourth generation communications technologies, a CA (Carrier Aggregation, carrier aggregation) technology, also known as a spectrum aggregation (Spectrum Aggregation) technology or a bandwidth extension (Bandwidth Extension) technology, is introduced. In the carrier aggregation technology, spectrums of two or more component carriers (Component Carrier) are aggregated to obtain wider transmission bandwidth, where the spectrums of the component carriers may be contiguous continuous spectrums, or may be non-contiguous spectrums in a same frequency band or even discontinuous spectrums in different frequency bands. An LTE Rel-8/9 UE (User Equipment, user equipment) can access only one of the component carriers to receive and send data, but an LTE-A UE can simultaneously access multiple component carriers according to a capability of the LTE-A UE and a service requirement to receive and send data.
To support hybrid automatic repeat, a UE needs to feed back an HARQ-ACK (Hybrid Automatic Repeat request-Acknowledgement, hybrid automatic repeat request acknowledgement) to a base station through a PUCCH (Physical Uplink Control Channel, physical uplink control channel) and a PUSCH (Physical Uplink Shared Channel, physical uplink shared channel), where the hybrid automatic repeat request acknowledgement may also be simply referred to as an ACK (Acknowledgement, acknowledgement)/NACK (Negative Acknowledgement, negative acknowledgement).
In an existing CA system, duplex modes of aggregated carriers are the same, for example, the duplex modes may all be FDD (Frequency Division Duplex, frequency division duplex) or may all be TDD (Time Division Duplex, time division duplex), but in a follow-up LTE system, the duplex modes may evolve to aggregation of different duplex modes, that is, the duplex modes of the aggregated carriers may be different. For example, duplex modes of some carriers are FDD, but duplex modes of other carriers are TDD. In a follow-up LTE system, the duplex modes may also evolve to a mode in which a primary component carrier in aggregated carriers is a carrier whose duplex mode is time division duplex TDD, and a secondary component carrier is a supplemental (Supplemental) downlink carrier, for example, the supplemental downlink carrier may indicate that all subframes on the carrier are downlink subframes. In the existing CA system, an HARQ-ACK is sent only on a primary component carrier. In this evolution direction, if the primary component carrier is a TDD carrier, an HARQ-ACK corresponding to an FDD carrier or a supplemental downlink carrier also needs to be fed back on the TDD carrier. The supplemental downlink carrier may also be referred to as a supplemental downlink serving cell.
However, in an existing LTE system, for an FDD carrier, HARQ-ACK timing is n+4, that is, an HARQ-ACK corresponding to a PDSCH (Physical Downlink Shared Channel, physical downlink shared channel) transmitted in a downlink subframe n is fed back in an uplink subframe n+4. However, if an HARQ-ACK corresponding to an FDD carrier or a supplemental downlink carrier is fed back on a TDD carrier, because on the TDD carrier, only some subframes of one radio frame are used for uplink transmission, if existing FDD HARQ-ACK timing is used, downlink subframes of some FDD carriers or downlink subframes of supplemental downlink carriers have no corresponding uplink subframe in which an HARQ-ACK is fed back, as a result, these downlink subframes cannot be used to schedule downlink data, which results in waste of resources. Therefore, in this case, an HARQ-ACK transmission mechanism of an FDD serving cell or an HARQ-ACK transmission mechanism of a supplemental downlink serving cell needs to be redesigned.